upon a healthy, balanced natural environment and that aquaculturists have a vested interest in assuring that the marine environment is not degraded. Many different toxic algae have been implicated in fish kills, although in most cases the precise toxins or mechanisms of toxicity are unclear. Nonetheless, many different types of toxins have been isolated from various algae and many of these are ichthyotoxic. Neurotoxins are especially common. Clinical signs of algal neurotoxicity include disorientation, loss of equilibrium, and sporadic hyperactivity. Other algae mechanically obstruct or damage the gills of fishes, causing hypoxia. Dinoflagellates represent the predominant toxin producing algae. Many dinoflagellates have been implicated or suspected in fish kills. The red tide dinoflagellate (Gymnodinium breve) causes mass mortalities of fish and invertebrates in states bordering the Gulf of Mexico. Some dinoflagellate toxins are transferred up the food chain (e.g., Alexandrium) and have caused mortalities in wild fish (e.g., Atlantic herring Clupea harengus) that consume tainted zooplankton along the northwest Atlantic coast of the United States. Other algal blooms associated with toxicity include the Prymnesium parvum, which causes mortality in brackishwater and marine pond fish (e.g., mullet Mugil spp.) in Europe and the Middle East. Diatoms of the genus Chaetoceros have been associated with mortality in seawater-cultured salmonids. The spines of other alga apparently cause it to become lodged on or in the gills. Diatoms may become embedded in gill tissue, inciting a foreign body reaction. Epithelial hyperplasia causes hypoxia. In some cases, hyperactive mucus production appears to be primarily responsible for the hypoxia. Other algae have been less commonly associated with ichthyotoxicity, although there is considerable speculation about possible sublethal effects. The most well recognized economic impact of toxic algae is seafood contamination by their toxins (Table 11-2). This has caused major economic losses in shellfish, including cultured species. Other toxic algae are known to cause serious illness in persons exposed to their toxins. Thus, the presence of toxic algae poses a serious threat to the future success of marine aquaculture, and there is a need to have intensified efforts to address these problems. The price of fish and invertebrates has dropped as a result of red tide scare, particularly in the Philippines. Prices of fishes caught from other areas free from red tide contamination have also been affected by the red tide scare.
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Health Management in Aquaculture